Responses of atrium and ventricle sympathetic nerve stimulation
to sustained
MOTOYUKI FURUYAMA, TAKASHI HANEDA, JUN IKEDA, TETSUYA HIRAMOTO, TOSHIAKI SAKUMA, HITOSHI KANDA, KUNIO SHIRATO, AND TAMOTSU TAKISHIMA The First Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Miyagi 980, Japan
FURUYAMA, MOTOYUKI, TAKASHI HANEDA, JUN IKEDA, TETSUYA HIRAMOTO, TOSHIAKI SAKUMA, HITOSHI KANDA, KUNIO SHIRATO, AND TAMOTSU TAKISHIMA. Responses of atrium and ventricle to sustained sympathetic nerve stimulation. Am. J. Physiol. 261 (Heart Circ. Physiol. 30): H1889-H1894, 1991.-To determine whether chonotropic and atria1 inotropic responses to sympathetic nerve stimulation are maintained longer than ventricular inotropic response, the present study was performed with control and acute reserpinized dogs. We stimulated the right stellate ganglion of both groups supramaximally for 60 min and compared right atria1 responses (chronotropism and inotropism) with left ventricular (LV) dP/dt,,,. In the control group, heart rate (HR) immediately increased and was only slightly attenuated with 60 min of stimulation, and right atria1 (RA) inotropic response was less attenuated than was LV response (7% in HR, 33% in RA dP/dt,,,, 50% in LV dP/dt,,,, P < 0.01, from the peak value of each response). RA and LV norepinephrine (NE) content was decreased by the stimulation but remained higher than the LV control value. In the reserpinized group, NE content in the RA was low before the stimulation and was further decreased by the stimulation. In this group, HR response was attenuated (27% in HR, P < 0.01) as was LV dP/dt,,,, and the difference in contractile responsiveness between atrium and ventricle disappeared (58% in RA dP/dt,,, vs. 61% in LV dP/dt,,,, NS). The results indicate that the chronotropic response was only slightly attenuated and that the atria1 contractile response was attenuated less than the ventricular response, with sustained sympathetic nerve stimulation in the normal heart. This can be ascribed to the much higher NE content in the RA than that in the LV.
atria1 contractility respond in a similar fashion to sustained sympathetic nerve stimulation. From the viewpoint of the disparity in myocardial NE, sympathetic support to the atrium and to the ventricle are expected to differ. However, to our knowledge, no one has investigated the difference in time course between chronotropic and inotropic responses during sustained sympathetic nerve stimulation with supramaximal intensity. Similarly, the difference between atria1 and ventricular contractile responses to the same stimulation, as described above, has not been clarified. Since sympathetic innervation to the right atrium (RA) is known to be richer than that to the LV, myocardial NE content of the RA is greater than that of the LV (1, 10, 15). Therefore, it may be assumed that the functional capacity of the cardiac sympathetic nerve ending for sustained stimulation is greater in the RA than in the ventricle. Therefore, we hypothesized that HR and atria1 contractile response can be maintained longer compared with ventricular contractile response during stimulation. In this study, we stimulated the right stellate ganglion (RSG) supramaximally in experimental dogs for 60 min and compared the response of the RA (chronotropism and inotropism) with LV contractility. We also performed a similar experiment in acute reserpinized dogs with partially reduced myocardial NE content.
heart rate; atria1 contractile response; ventricular contractile response; sympathetic stimulation; myocardial norepinephrine
General Preparation
METHODS
The 37 mongrel dogs were anesthetized with pentobarbital sodium (30 mg/kg). The anesthetic was infused continuously (5 mg kg-’ . h-‘) to maintain stable anesthesia throughout the experiment. Unlike the majority of commonly used anesthetic agents, barbiturates do not cause an increase in plasma catecholamine concentration (7,14); pentobarbital sodium was therefore chosen as the anesthetic for this study. After intubation, artificial ventilation was accomplished with a Harvard respirator with fractional concentration of 02 in inspired gas (FIN,) of 40%, and arterial blood oxygen saturation was kept within 95-99% throughout the experiment. The arterial blood oxygen saturation was measured with a hemoxymeter (Radiometer a/s Copenhagen, type OSM-2). LV pressure was measured using a catheter-tip manometer with a fluid-filled lumen (8-Fr; Millar Instruments) through the femoral artery, and the maximum l
that supramaximal stimulation of the cardiac sympathetic nerve induces maximum ventricular contractile response within a minute, and the response then attenuates progressively with time (3, 15, 20, 21). We have reported that prolonged sympathetic stimulation attenuates the left ventricular (LV) contractile response progressively during 60 min of stimulation and that the attenuated response in contractility continued for at least 15 min after cessation of stimulation (8). This progressive attenuation in contractile response is thought to be due to acute depletion of a “secretory” pool of norepinephrine (NE) in the postganglionic sympathetic nerve terminals (2, 11). However, it is not clear whether heart rate (HR) and IT HAS BEEN OBSERVED
0363-6135/91
$1.50 Copyright
0 1991 the American
Physiological
Society
H1889
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (128.143.007.175) on November 23, 2018. Copyright © 1991 the American Physiological Society. All rights reserved.
H1890
CARDIAC
RESPONSE
TO
SUSTAINED
value of its first derivative (LV dP/dt,,,) was used as an index of ventricular contractility. A right-sided thoracotomy was performed at the third intercostal space. A pericardiotomy ~40 mm in length was performed on the auricle, and a small balloon was then inserted into the RA cavity through an incision at the tip of the atria1 appendage. The balloon was anchored at the tip of the auricle by a ligature. The balloon was filled with about 1 ml of saline (12, 13). The pressure in the atria1 balloon was measured using a catheter-tip manometer with a fluid-filled lumen (Millar Instruments). The volume of the balloon was determined so that the dP/dt of the balloon pressure reached the maximum value. The balloon volume used in this study was 0.7-1.2 ml (average 0.9 ml). The maximum value of the derivative (RA dP/ dt,,,) was used as an index of atria1 contractility (13) The balloon caused no interference with blood flow through the atria1 cavity, because the balloon was small and almost fully covered by the auricle. For comparison among animals, the data for each dog were normalized by taking the peak inotropic response as 100% and scaling all other data to this value. An electrocardiogram was recorded at the lead on which the P wave and QRS complex could be most clearly estimated. The RSG was carefully isolated, and bipolar silver electrodes were placed at the RSG and connected to an electrical stimulator (Nihon-Kohden, MSE-3R). Bilateral cervical vagi were cut at least 30 min before hemodynamic measurements were made. Experimental
Procedure
and Analysis
Control group. Eleven dogs were divided into two groups: five dogs were used for physiological measurement and six dogs were used for myocardial NE measurement. From the group of dogs, we first recorded electrocardiogram, LV pressure, dP/dt,,,, the pressure in the balloon, and RA dP/dt,,,. We then electrically stimulated the RSG for 60 min. The duration of the neural stimulus impulses was 1 ms, the interval was 100 ms, and the amplitude was 10 V. We considered this condition of stimulation as supramaximal according to former reports and our previous experiments (8, 22). Hemodynamic recordings were made at 1, 10, 30, and 60 min during sustained stimulation. With the six dogs, we performed biopsies and measured myocardial NE content. To exclude the effects of myocardial biopsy on hemodynamic measurements, we performed myocardial biopsies in other groups of dogs. In these groups, bilateral thoracotomy was performed at the right third intercostal space and the left fourth intercostal space. Before RSG stimulation, two incisions, each ~30 mm in length, were made in the pericardium, and, with small scissors, 20-40 mg of myocardial tissue was sampled from the RA appendage and the avascular area in the midportion of LV free wall. The wounds, along with small pieces of felt attached to either side of the wound, were sutured together with silk thread, and the pericardium was then sutured loosely. Biopsy specimens were frozen in ice-cold saline soon after biopsy. We then stimulated the RSG for 60 min, using the method described above. Immediately after the cessation of stimu-
SYMPATHETIC
STIMULATION
lation, we sampled the myocardium again from an area near the previous sampling site. We measured the myocardial NE content and estimated the change caused by sustained RSG stimulation. In four of these six dogs, the area including the sinus node was also sampled after RSG stimulation for comparison of myocardial NE content. These biopsy specimens were homogenized with 10 ml of 0.1 N HCl at 4°C by use of an ultra-dispersor at 3,000 rpm for 10 min. The supernatant was kept at -10°C until assay was performed. NE assay was carried out with high-pressure liquid chromatography-electrochemical analysis within 2 wk of sampling (Waters model 501 pump, ESA Coulochem model 5100A analyzer). The limit of detection of this assay is -30 pg, and coefficients of variations were ~14% (5). These coefficients of variations refer to the percentage variation of each assay from the popul .ation mean in a normal distribution of assays in the set of plasma (5). Reserpine-heated group. Eleven dogs were divided into two groups: five dogs were used for physiological measurement and six dogs for myocardial NE measurement. In this group, we administered reserpine (3 mg/kg) 3 h before RSG stimulation. Hemodynamic measurement and myocardial sampling were performed in the same manner and according to the same schedule as that described for the control group. In an additional six dogs, we studied the direct effect of reserpine on myocardial NE change in the RA appendage and the LV free wall 3 and 4 h after reserpine administration without stimulation. Repetitive l- min stimulation group. To determine the direct effect of reserpine on HR response, we administered reserpine (3 mg/kg) to three mongrel dogs and stimulated RSG repetitively for 1 min at 3 and 4 h after reserpine administration. Dobutamine group. To determ .ine whether the desensitization of myocardial ,&adrenoceptors and cardiac muscle fatigue are related to this phenomenon, we injected dobutamine intravenously (15 pg. kg-‘. min-‘; n = 6) for 60 min and compared the responses of HR, RA and LV dP/dt,,,. Hemodynamic recordings dWknax, were made at 10, 30, and 60 min during continuous dobutamine infusion. Results are expressed as means t SE. Significance was determined using the Student’s t test for paired and unpaired data, and the significance level was taken as P c 0.05. RESULTS
Table 1 shows the hemodynamic parameters of both controls and reserpine-treated groups before stim ulation. Hemodynamic parameters did not differ between the two groups before stimulation, except for HR. Figure 1A shows the time course of HR, RA dP/dt,,,, and LV dP/dt,,, with RSG stimulation in the control group. HR increased from 120 t 7 (SE) beats/min to 211 t 9 beats/min (P < 0.01) in 1 min and remained nearly constant throughout the period of stimulation, and the HR at 60 min was 196 t 9 beats/min. RA dP/dt,,, increased from 443 t 70 to 1,185 t 199 mmHg/s (P
to sustained
MOTOYUKI FURUYAMA, TAKASHI HANEDA, JUN IKEDA, TETSUYA HIRAMOTO, TOSHIAKI SAKUMA, HITOSHI KANDA, KUNIO SHIRATO, AND TAMOTSU TAKISHIMA The First Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Miyagi 980, Japan
FURUYAMA, MOTOYUKI, TAKASHI HANEDA, JUN IKEDA, TETSUYA HIRAMOTO, TOSHIAKI SAKUMA, HITOSHI KANDA, KUNIO SHIRATO, AND TAMOTSU TAKISHIMA. Responses of atrium and ventricle to sustained sympathetic nerve stimulation. Am. J. Physiol. 261 (Heart Circ. Physiol. 30): H1889-H1894, 1991.-To determine whether chonotropic and atria1 inotropic responses to sympathetic nerve stimulation are maintained longer than ventricular inotropic response, the present study was performed with control and acute reserpinized dogs. We stimulated the right stellate ganglion of both groups supramaximally for 60 min and compared right atria1 responses (chronotropism and inotropism) with left ventricular (LV) dP/dt,,,. In the control group, heart rate (HR) immediately increased and was only slightly attenuated with 60 min of stimulation, and right atria1 (RA) inotropic response was less attenuated than was LV response (7% in HR, 33% in RA dP/dt,,,, 50% in LV dP/dt,,,, P < 0.01, from the peak value of each response). RA and LV norepinephrine (NE) content was decreased by the stimulation but remained higher than the LV control value. In the reserpinized group, NE content in the RA was low before the stimulation and was further decreased by the stimulation. In this group, HR response was attenuated (27% in HR, P < 0.01) as was LV dP/dt,,,, and the difference in contractile responsiveness between atrium and ventricle disappeared (58% in RA dP/dt,,, vs. 61% in LV dP/dt,,,, NS). The results indicate that the chronotropic response was only slightly attenuated and that the atria1 contractile response was attenuated less than the ventricular response, with sustained sympathetic nerve stimulation in the normal heart. This can be ascribed to the much higher NE content in the RA than that in the LV.
atria1 contractility respond in a similar fashion to sustained sympathetic nerve stimulation. From the viewpoint of the disparity in myocardial NE, sympathetic support to the atrium and to the ventricle are expected to differ. However, to our knowledge, no one has investigated the difference in time course between chronotropic and inotropic responses during sustained sympathetic nerve stimulation with supramaximal intensity. Similarly, the difference between atria1 and ventricular contractile responses to the same stimulation, as described above, has not been clarified. Since sympathetic innervation to the right atrium (RA) is known to be richer than that to the LV, myocardial NE content of the RA is greater than that of the LV (1, 10, 15). Therefore, it may be assumed that the functional capacity of the cardiac sympathetic nerve ending for sustained stimulation is greater in the RA than in the ventricle. Therefore, we hypothesized that HR and atria1 contractile response can be maintained longer compared with ventricular contractile response during stimulation. In this study, we stimulated the right stellate ganglion (RSG) supramaximally in experimental dogs for 60 min and compared the response of the RA (chronotropism and inotropism) with LV contractility. We also performed a similar experiment in acute reserpinized dogs with partially reduced myocardial NE content.
heart rate; atria1 contractile response; ventricular contractile response; sympathetic stimulation; myocardial norepinephrine
General Preparation
METHODS
The 37 mongrel dogs were anesthetized with pentobarbital sodium (30 mg/kg). The anesthetic was infused continuously (5 mg kg-’ . h-‘) to maintain stable anesthesia throughout the experiment. Unlike the majority of commonly used anesthetic agents, barbiturates do not cause an increase in plasma catecholamine concentration (7,14); pentobarbital sodium was therefore chosen as the anesthetic for this study. After intubation, artificial ventilation was accomplished with a Harvard respirator with fractional concentration of 02 in inspired gas (FIN,) of 40%, and arterial blood oxygen saturation was kept within 95-99% throughout the experiment. The arterial blood oxygen saturation was measured with a hemoxymeter (Radiometer a/s Copenhagen, type OSM-2). LV pressure was measured using a catheter-tip manometer with a fluid-filled lumen (8-Fr; Millar Instruments) through the femoral artery, and the maximum l
that supramaximal stimulation of the cardiac sympathetic nerve induces maximum ventricular contractile response within a minute, and the response then attenuates progressively with time (3, 15, 20, 21). We have reported that prolonged sympathetic stimulation attenuates the left ventricular (LV) contractile response progressively during 60 min of stimulation and that the attenuated response in contractility continued for at least 15 min after cessation of stimulation (8). This progressive attenuation in contractile response is thought to be due to acute depletion of a “secretory” pool of norepinephrine (NE) in the postganglionic sympathetic nerve terminals (2, 11). However, it is not clear whether heart rate (HR) and IT HAS BEEN OBSERVED
0363-6135/91
$1.50 Copyright
0 1991 the American
Physiological
Society
H1889
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (128.143.007.175) on November 23, 2018. Copyright © 1991 the American Physiological Society. All rights reserved.
H1890
CARDIAC
RESPONSE
TO
SUSTAINED
value of its first derivative (LV dP/dt,,,) was used as an index of ventricular contractility. A right-sided thoracotomy was performed at the third intercostal space. A pericardiotomy ~40 mm in length was performed on the auricle, and a small balloon was then inserted into the RA cavity through an incision at the tip of the atria1 appendage. The balloon was anchored at the tip of the auricle by a ligature. The balloon was filled with about 1 ml of saline (12, 13). The pressure in the atria1 balloon was measured using a catheter-tip manometer with a fluid-filled lumen (Millar Instruments). The volume of the balloon was determined so that the dP/dt of the balloon pressure reached the maximum value. The balloon volume used in this study was 0.7-1.2 ml (average 0.9 ml). The maximum value of the derivative (RA dP/ dt,,,) was used as an index of atria1 contractility (13) The balloon caused no interference with blood flow through the atria1 cavity, because the balloon was small and almost fully covered by the auricle. For comparison among animals, the data for each dog were normalized by taking the peak inotropic response as 100% and scaling all other data to this value. An electrocardiogram was recorded at the lead on which the P wave and QRS complex could be most clearly estimated. The RSG was carefully isolated, and bipolar silver electrodes were placed at the RSG and connected to an electrical stimulator (Nihon-Kohden, MSE-3R). Bilateral cervical vagi were cut at least 30 min before hemodynamic measurements were made. Experimental
Procedure
and Analysis
Control group. Eleven dogs were divided into two groups: five dogs were used for physiological measurement and six dogs were used for myocardial NE measurement. From the group of dogs, we first recorded electrocardiogram, LV pressure, dP/dt,,,, the pressure in the balloon, and RA dP/dt,,,. We then electrically stimulated the RSG for 60 min. The duration of the neural stimulus impulses was 1 ms, the interval was 100 ms, and the amplitude was 10 V. We considered this condition of stimulation as supramaximal according to former reports and our previous experiments (8, 22). Hemodynamic recordings were made at 1, 10, 30, and 60 min during sustained stimulation. With the six dogs, we performed biopsies and measured myocardial NE content. To exclude the effects of myocardial biopsy on hemodynamic measurements, we performed myocardial biopsies in other groups of dogs. In these groups, bilateral thoracotomy was performed at the right third intercostal space and the left fourth intercostal space. Before RSG stimulation, two incisions, each ~30 mm in length, were made in the pericardium, and, with small scissors, 20-40 mg of myocardial tissue was sampled from the RA appendage and the avascular area in the midportion of LV free wall. The wounds, along with small pieces of felt attached to either side of the wound, were sutured together with silk thread, and the pericardium was then sutured loosely. Biopsy specimens were frozen in ice-cold saline soon after biopsy. We then stimulated the RSG for 60 min, using the method described above. Immediately after the cessation of stimu-
SYMPATHETIC
STIMULATION
lation, we sampled the myocardium again from an area near the previous sampling site. We measured the myocardial NE content and estimated the change caused by sustained RSG stimulation. In four of these six dogs, the area including the sinus node was also sampled after RSG stimulation for comparison of myocardial NE content. These biopsy specimens were homogenized with 10 ml of 0.1 N HCl at 4°C by use of an ultra-dispersor at 3,000 rpm for 10 min. The supernatant was kept at -10°C until assay was performed. NE assay was carried out with high-pressure liquid chromatography-electrochemical analysis within 2 wk of sampling (Waters model 501 pump, ESA Coulochem model 5100A analyzer). The limit of detection of this assay is -30 pg, and coefficients of variations were ~14% (5). These coefficients of variations refer to the percentage variation of each assay from the popul .ation mean in a normal distribution of assays in the set of plasma (5). Reserpine-heated group. Eleven dogs were divided into two groups: five dogs were used for physiological measurement and six dogs for myocardial NE measurement. In this group, we administered reserpine (3 mg/kg) 3 h before RSG stimulation. Hemodynamic measurement and myocardial sampling were performed in the same manner and according to the same schedule as that described for the control group. In an additional six dogs, we studied the direct effect of reserpine on myocardial NE change in the RA appendage and the LV free wall 3 and 4 h after reserpine administration without stimulation. Repetitive l- min stimulation group. To determine the direct effect of reserpine on HR response, we administered reserpine (3 mg/kg) to three mongrel dogs and stimulated RSG repetitively for 1 min at 3 and 4 h after reserpine administration. Dobutamine group. To determ .ine whether the desensitization of myocardial ,&adrenoceptors and cardiac muscle fatigue are related to this phenomenon, we injected dobutamine intravenously (15 pg. kg-‘. min-‘; n = 6) for 60 min and compared the responses of HR, RA and LV dP/dt,,,. Hemodynamic recordings dWknax, were made at 10, 30, and 60 min during continuous dobutamine infusion. Results are expressed as means t SE. Significance was determined using the Student’s t test for paired and unpaired data, and the significance level was taken as P c 0.05. RESULTS
Table 1 shows the hemodynamic parameters of both controls and reserpine-treated groups before stim ulation. Hemodynamic parameters did not differ between the two groups before stimulation, except for HR. Figure 1A shows the time course of HR, RA dP/dt,,,, and LV dP/dt,,, with RSG stimulation in the control group. HR increased from 120 t 7 (SE) beats/min to 211 t 9 beats/min (P < 0.01) in 1 min and remained nearly constant throughout the period of stimulation, and the HR at 60 min was 196 t 9 beats/min. RA dP/dt,,, increased from 443 t 70 to 1,185 t 199 mmHg/s (P
